Reducing Stormwater Runoff Impacts

The primary stormwater conveyance system in this portion of the butte consists of sheet (overland) flow to a series of collection channels and culverts conveying stormwater runoff north towards the 158th Avenue Access Road. A collection channel along the south side of this access road conveys runoff in a westerly direction. A portion of this runoff is diverted through several culverts (Figure 3) and is discharged through open outfalls into MMHP. Runoff is conveyed from these outfalls via overland flow towards MMHP (Figures 4 and 5), where the flooding has occurred in the past. The remaining runoff is conveyed along the access road collection channel in a westerly direction towards the 158th Avenue entrance to the park. The area contributing runoff includes approximately 1.2 acres of impervious surface, which consists of existing reservoirs and associated crushed-rock driving surfaces.

In a “good faith” effort to help mitigate the impacts to downstream properties, PWB retained Kennedy/Jenks Consultants to provide design services for reducing the impact to these downstream properties. The design of the facility would focus on the following critical issues:

Reduce the impact to downstream properties by re-directing existing runoff in a westerly direction toward a new surface infiltration facility west of the SE 158th Avenue access road; and

Size and design the proposed infiltration facility in accordance with the requirements of the City of Portland Bureau of Environmental Services’ Stormwater Management Manual (Manual).

Regulatory requirements
As the city of Portland is developed, impervious surfaces create increased amounts of stormwater runoff during rainfall events, disrupting the natural hydrologic cycle. Without stormwater management, these conditions erode stream channels, prevent groundwater recharge and often result in combined sewer overflows (CSOs) and basement sewer back-ups. Parking lots, roadways, rooftops and other impervious surfaces increase the pollution levels and temperature of stormwater that is transported to streams, rivers and groundwater resources. Implementing measures to mitigate the impacts of increased runoff will help protect Portland’s water resources, which in turn will provide great benefit to human health, fish and wildlife habitat, recreational resources and drinking water.